Protective surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives, where the polymeric binder acts as a fluid vehicle for the pigments and imparts rheological properties to the fluid paint coating. Upon curing, the polymeric binder hardens and functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some paint coatings contain little or no opacifying pigments and are described as clear coatings. The manufacture of paint coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in the polymeric binder, and thinning to commercial standards.
Epoxy resins are particularly desirable for use in surface coating materials as a vehicle or polymeric binder to advantageously provide toughness, flexibility, adhesion, and chemical resistance to the applied coating film. Hence, water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions. Coatings for soft drink and beer cans, for instance, are critical due to taste sensitivity and must not alter the taste of canned beverages. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by absorption of flavor by the coating, or sometimes by chemical reaction, or by some combination thereof.
In commonly assigned U.S. Pat. No. 4,212,781, a process is disclosed for modifying epoxy resin by reacting the epoxy resin with addition polymerizable ethylenic monomer in the presence of at least 3% by weight of benzoyl peroxide (or the free radical initiating equivalent thereof) based on monomer at a suitable reaction temperature to produce a reaction mixture comprising an epoxy-acrylic copolymer mixture containing epoxy resin, graft epoxy-acrylic polymer, and associatively-formed ungrafted addition polymer. The in-situ polymerized monomers include acid functional monomers to provide acid functionality in the reaction mixture sufficiently high to effect stable dispersion of the resulting reaction product in a basic aqueous medium. Related commonly assigned U.S. Pat. No. 4,285,847 pertains to dispersing the preformed epoxy-acrylic copolymer mixture into water followed by in-situ emulsion copolymerization of ethylenic monomers in a second step polymerization.
Ordinarily the epoxy resin content of can coatings needs to be high to obtain good film integrity properties such as blush resistance and odor absorption. Beer and soft drink beverage can manufacturers require coatings which also need flexibility at lower bake temperatures and invariably require soft Tg ethylenic monomers such as ethyl acrylate, butyl acrylate and 2-ethyl hexyl acrylate. However, the soft monomers tend to cause a problem with carbon dioxide in beverages where excessive carbon dioxide losses from the beverage can cause a gushing effect when the beverage can is opened causing a messy eruption of beverage.
It now has been found that minor amounts of divinyl benzene included in the second step emulsion copolymerization of monoethylenically unsaturated monomers considerably improves the gush resistance and provides improved adhesion properties.
In accordance with this invention, epoxy resin is first grafted with ethylenic monomers (including carboxyl monomers) copolymerized in the presence of the epoxy resin and in the absence of water. A preformed epoxy-graft-acrylic resin mixture is preferably formed as proposed in commonly assigned U.S. Pat. No. 4,212,781. The preformed epoxy-graft-acrylic resin mixture is then dispersed into water with assistance of amines or other basic materials to produce an aqueous dispersion of the preformed epoxy-graft-acrylic. In accordance with this invention, second stage mono-ethylenically unsaturated monomers containing minor amounts of divinyl benzene are copolymerized with the aqueous dispersed preformed acrylic-graft-epoxy to produce moderately crosslinked emulsion microgel particles. The divinyl benzene crosslinking improves film properties and particularly eliminates the carbon dioxide gassing in carbon dioxide type beverages. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and illustrative examples.